(CBS/AP) Mitchell Johnson, one of the two boys convicted of what was at the time the worst school shooting on record, gets to walk away Thursday scot-free, reports CBS News Correspondent Lee Cowan (video).

It was just seven years ago, when Johnson, then 13, and his companion, 11-year-old Andrew Golden, stole an arsenal of weapons and then opened fire on a courtyard full of students and teachers at an Arkansas middle school.

Their ambush-style assault killed four young girls and an English teacher, who tried to shield her students from the barrage of bullets.

Under a now-changed Arkansas law, the boys were charged as delinquents, which meant they could only be held until they were 18. Federal prosecutors then stepped in with firearms violations which kept them in prison for three more years. But Johnson turns 21 on Thursday and cannot legally be held any longer.

Many Jonesboro residents question whether justice has been served.

"They barely served a year and a half for each life they took, whereas some people kill one person and serve the rest of their lives. It's really unfair," said Brandi Varner, who lost her younger sister Brittheny in the attack.

Student Whitney Irving was shot in the back, but survived. Although she has since graduated from high school, married and had a child, the attack remains a part of her everyday life.

"Not one day has gone by in seven years that I have not had a thought about what happened," she told CBS News' The Early Show.

Mitchell Wright, whose wife, Shannon, was the teacher killed in the attack, told CBS that "seven years later, it's like the closure that you had is totally wiped away because there's no justice in this whatsoever.

Wright's son Zane was two at the time of the attack. He's now nine and says, "I don't want him to get out of prison, but I realize that he's going to have to get out of prison. And it's just not fair because he gets to go home and see his mom, start a new life. And my mom, she doesn't."

On March 24, 1998, Johnson and Golden stole high-powered rifles from Golden's grandfather. Dressed in camouflage, they waited in the woods behind the school until the lunch hour, when Golden ran into a hallway to trigger a fire alarm.